The present invention concerns an integrated variable electrical capacitance device and a method for collectively producing said devices.
The term variable electrical capacitance device is taken to mean any device or component that comprises one or several chip capacitors whose electrical capacitance can be modified.
Such devices are used, for example, for producing resonant filters that one wishes to tune to different frequencies.
A specific application of variable capacitance devices is producing telecommunications equipment by radio relay channels. The integrated character of variable capacitances is dictated by the imperatives of miniaturisation of the equipment concerned.
A description of the prior art is given in the document (1) whose references are detailed at the end of the description. Additional information may be found in documents (2) to (4) whose references are also detailed at the end of the description.
An integrated electrical capacitance generally comprises a fixed armature and a mobile armature arranged facing each other, and mutually insulated. The mobile armature may be formed or supported by a membrane. The mobility of the armature may result from the flexible and deformable character of the membrane, or due to the fact that said membrane is linked to a support fastened via flexible means, such as small joists.
The space separating the armatures may be a vacuum or may contain a gas. The variable capacitance is generally associated with control drive means. This makes it possible to displace the mobile armature in relation to the fixed armature. The displacement of the mobile armature has the effect of modifying the distance separating the facing armatures and thus modifying the capacitance of the capacitor formed by the armatures.
In essence, three types of drive means that can be used for controlling the displacement of the mobile armature are known. Thus one distinguishes electrostatic means, piezoelectric means and differential expansion thermal means (bimetallic strip).
Among these means, electrostatic control is generally preferred. This type of control is preferred for the simplicity of its production and for its reduced power consumption. Electrostatic drive means use electrodes between which one generates electrostatic forces by applying a potential difference. The forces are profitably employed for displacing the mobile armature.
However, with highly miniaturised devices, an important difficulty arises in supplying the control voltage required for the exercise of the electrostatic forces. This is around 10 to 100 volts.
A possible solution for limiting the control voltage consists in making the joists that maintain the mobile armature more flexible. Improved joist flexibility makes it possible, in fact, to use a lower control voltage.
On the other hand, it also increases the sensitivity of the device to accelerations. In this respect, the reader may refer to documents (2) and (3), which propose profitably using the sensitivity of a mobile armature to acceleration to the production of an accelerometer.
However, in the devices covered by the present description, the accelerations, including that of gravity, have the effect of displacing the mobile armature and generating uncontrollable variations in the capacitance. This phenomenon is particularly troublesome in portable equipment, subjected to strong and repeated accelerations.
This undesirable effect may be reduced by using mobile armatures in the form of very thin membranes, or borne by such membranes. Their light weight makes them, in fact, less sensitive to acceleration.
However, the use of thin membranes causes other difficulties. For example, the resonance modes of the membranes, at low frequency, can generate capacitance instabilities. Moreover, very thin membranes are fragile and subject to deformation over time. The result, in particular, is a capacitance drift at rest, in other words capacitance drift in the absence of an applied electrical field.
The aim of the present invention is to propose an integrated variable capacitance device that does not have the above limitations and difficulties.
A particular aim is to propose such a device that has low sensitivity to accelerations and which has, nevertheless, good stability over time, while at the same time having a low electricity consumption and which is capable of being controlled by a low voltage.
A further aim is to propose a reliable device capable of being used in portable telecommunications equipment.
A yet further aim is to propose a method for producing said device.
Finally, an aim of the invention is to propose a method suitable for the collective and concomitant production of a large number of variable capacitance devices.
In order to achieve these aims, the invention concerns more specifically an integrated variable capacitance device comprising at least one membrane forming at least one mobile armature and having at least one principal face facing at least one fixed armature. According to the invention, the membrane is integral with at least one rigidity rib lying in a perpendicular direction to said principal face. Preferably, the rib is as thin as possible in a direction parallel to the principal face.
One considers that the membrane forms a mobile armature when at least one part of the membrane, which can be brought nearer or moved further away from the fixed armature, is in a conductive material, or when the membrane is provided in this part with a layer of conductive material forming one or several armatures. In other words, at least one part of the membrane constitutes or bears an armature.
One considers that the membrane is in a conductive material when said material conducts sufficient electricity to constitute a condenser armature. This does not exclude choosing semi-conductor materials.
The displacement of the mobile armature can take place by deforming the membrane, which then has at least one flexible part, and/or by deforming one or several joists suspending the membrane.
The rib makes it possible to confer to the membrane, and thus to the mobile armature, a good rigidity despite its thinness and thus its lightness. It thereby makes it possible to use thinner membranes than in known devices. The membrane, thus rigidified, has better resistance over time, better stability and at the same time is relatively insensitive to accelerations.
The rib is situated on at least one of the principal faces, in order words one of the faces with the largest surface. This characteristic makes it possible to optimise the stiffening function of the rib(s), but does not prejudice either the shape or the orientation of the membrane.
According to a specific aspect of the invention, the device may comprise a substrate bearing the membrane and forming one or several fixed armatures.
Here again, one considers that the substrate forms a fixed armature when at least one part of the substrate is in a conductive material that can constitute an armature, or when the substrate is provided with one or several conductive layers constituting electrodes and thus forming the fixed armature(s).
In a specific embodiment, the device of the invention may comprise a first part arranged facing a first principal face of the membrane and a second part, integral with the first part, and arranged facing a second principal face of the membrane, at least one of the first and second parts forming a fixed armature.
In the case where the membrane is a flat membrane, the substrate may comprise two fixed armatures, arranged on either side of the membrane, facing its two principal faces. Advantageously, one of the fixed armatures may be used jointly with the mobile armature to form an electrostatic control drive means for the displacement of the membrane. The second fixed armature may then be reserved exclusively for constituting a variable capacitance, with the mobile armature.
It should be pointed out that a same set of fixed and mobile armatures may also be used at the same time as variable capacitance and as electrostatic control drive.
The invention also concerns a method for producing a variable capacitance device in which:
a) One provides a substrate having at least one first thin armature layer linked to a solid part by a buried insulating layer,
b) One forms on the first thin armature layer, an etching barrier layer, said etching barrier layer having at least one coupling opening leading into a part of the thin armature layer intended to form a mobile armature,
c) One forms at least one rigidity layer covering the etching barrier layer, and coming into contact with the first thin armature layer in the coupling opening,
d) One carries out a selective etching of the rigidity layer in order to form at least one rigidity rib extending out above the coupling opening, and on the part of the thin armature layer intended to form a mobile armature,
e) One liberates said part by local elimination of the etching barrier layer and the buried insulating layer, in order to form the mobile armature.
Armature layer is taken to mean a layer in conductive, semi-conductive or resistant material, able to constitute one or several condenser armatures.
In one specific embodiment of the method, one can use an etching barrier layer in an electrically insulating material, and a rigidity layer in an electrically conductive material. One thereby preserves, during step d), a part of the rigidity layer extending out above the part of the thin armature layer intended to form a mobile armature, and one mutually liberates said parts of the rigidity layer and the thin armature layer.
The liberation of the armature layer from the rigidity layer makes it possible to form the membrane. This can be done, for example, by local elimination of a sacrificial intercalated layer. This aspect is described in more detail in the text that follows.
The substrate, or more precisely the solid part of the substrate, may be used as a fixed armature, when it is not insulating.
Alternatively, one can also provide, during step a), a substrate that has, moreover, under the first armature layer, a second buried armature layer, electrically insulated from the first armature layer. In this case, one can preserve at least a part of this layer facing the mobile armature, to form a fixed armature.
According to yet another possibility, one can form in the solid part of the substrate a conductive zone forming a fixed armature by ion implantation.
The selective elimination of a sacrificial part of the buried layer may be carried out by making one or several access openings to this layer in order to submit it to etching agents. This operation may be made a great deal simpler when one provides, before the step b), one or several etching openings in the first armature layer.
Other characteristics and advantages of the invention will be made clear from the description that follows, while referring to the appended figures. This description is given solely by way of indication and is in nowise limitative.